Boromir had to die so that his antithesis, Faramir, could have his chance. It is fascinating construction, on Tolkien's part, to think of placing the Boromir character where he did. Had Boromir not startled Frodo with his open lust for the ring, would Frodo have ever decided to set off on his own? The seed of conflict had to be placed in the Fellowship, for otherwise it could not have been broken when it truly needed to be. Had it not been broken could Saruman have been dealt with effectively? Who would have gotten the Ents to rise up ? (I'm getting ahead again). Boromir was there to break up the Fellowship (Elrond's wisdom?) when it had to happen for the quest to continue.